Know anyone with a good scan tool? It will be able to see the stored codes and why it's not working. A bi-directional tool can test the dash switch and the actuators on the axle and transfer case too to make sure they're working right.

You need a good scan tool Mike. Essential to work on these things. If it were mine the first place I'd look is the fuel trims to see if there is a vacuum leak at the intake gaskets. Would also check out the fuel pump and injectors and coils.

You definitely want to get this hooked up to the proper scan tool, like a Tech-2, so you observe what's going on inside and see if the transmission has thrown any codes. If you join AllDataDIY, you can run through the troubleshooting guide with your symptoms.

If I could pay $1.10 for E85 that would make my city driving ten cents a mile and even less for the highway! 87 wouldn't come close at fourteen and a half cents a mile. Either way the motor runs soo much better on the stuff, it's a shame to switch back to gasoline and lose all that performance and smoothness!

This has been one of the most popular requests for a long time now. For some reason, we only see the bigger tanks for the diesels, everyone scared to death of what if liabilities. I have 31 and would like some more too!

That's correct and was addressed in the middle of the article.
I take offense to this Tom. You do not know me nor my motivations for this post. My name is the same all across the internet (including ebay) for better or worse, "swathdiver".

Sorry to hear this! Happened to me quite often when I used to own muscle cars and a jeep.
I had an immobilizer and a fuel pump cut off switch. So even if they defeated one, they would not get the other. One time, my brother left the immobilizer in valet mode and they got the car started but when the fuel lines ran out of fuel the car died and they abandoned it down the street from our home.
Your best bet would be some other form of visual deterrant, like sticker or flashing dash light or seeing the VIN etched in all the glass. Stuff like that. But those knuckleheads, didn't even see your dash cam, so you never know.
Some alarms, like a viper I think, will alert your remote. This guy knows a lot more than me about these kinds of things: https://www.tahoeyukonforum.com/members/doubeleive.64770/

Gruven is actually taking pre-orders to make these from brass.
https://www.tahoeyukonforum.com/threads/gruven-heater-core-t-connectors.107087/
https://www.tahoeyukonforum.com/threads/billet-t-connectors-for-heater-cores.107406/
http://www.gruvenparts.com/shop-by-car/gm-truck

The Case for the Tech-2
By J. A. Burns, Port Saint Lucie, Florida
I first learned the complete necessity of owning a scan tool designed for my given car at the dawn of the computerized automobile, owning and racing a stable of turbocharged Buick Regals. Even only being able to see and work with 20-40 sensor parameters proved crucial to keep our engines in top form.
Years later my wife and I bought a brand-new lemon from GM. That thing was in the shop all the time with gremlins that only the most dedicated mechanic could figure out. I wanted to buy a Tech-2 and work on it myself but they were too expensive at the time. When that one died, I budgeted a Tech-2 along with my next purchase of a GM product. It quickly paid for itself and keeps doing so. It’s the most important tool in my toolbox as it first determines which of my other tools, if any, will get used next!
So, what does this thing do? Well for starters, it can see everything! It can test almost everything. Have a misfire and want to turn off a coil pack or injector? Use the Tech-2. Alternator not working right? Power it up or down with the Tech-2. Are your catalytic converters acting up? Run a graph of the O2 sensors for a short drive and play it back when you return home. Hard to start? See that the fuel pump is at the commanded pressure. Air conditioning acting up? Use the Tech2 to check for codes and command the actuators back into position or a proper reset. It can also monitor the high and low pressures. Want to bleed your brakes thoroughly? The Tech-2 will bleed the ABS module correctly. It goes on and on and on. You can turn lights on and off, open and close windows and door locks, program tire pressures, enable Afterblow and see how many rpms the transmission is slipping and even how much torque the engine is making. You can see line pressures and even test the “valve body” or get a stuck piston unstuck; maybe! Same for the front axle and transfer case. Need to add some key fobs? The Tech-2 can do that too!
Of course, this machine cannot and should not be used by itself. It was designed to be used in conjunction with the shop manual which tells you how to use the Tech-2 and what to look for and do. That can be obtained via a subscription to AllDataDIY or one can buy the books on a disc or download them from SeriousCTBuyer on eBay. I like having both options and All Data is continually updated. The actual books are too expensive for me.
Another feature I didn’t even touch on yet is programming. Let’s say your instrument cluster fails and you buy a new or used one. It has to be programmed to your VIN (new) with the mileage and hours and this you can do yourself by buying access to GMs Tis2Web service. If you see that there’s an update for your transmission or you want to add side blind zone alert, you can do this without going to the dealership. If you change your engine from a 5.3 to a 6.2, you can program the computer for that but you cannot disable AFM. For that you’d need something like EFILive! or HPTuners. These aforementioned laptop-based products are for customizing your engine and transmission programs. I have yet to do any of this.
A quick word about generic code readers. These little Bluetooth ones that talk to an app on your phone are nice when you’re on the go. However, they do not always use the same codes to describe a problem that GM does. And most of them cannot talk to your ABS system or tell you why the heated seats stopped working. GM uses specific codes and for some items like the air actuators, provides symptom numbers which tell you why it failed, not just that it did. By the way, when the Yukon takes us on a trip, the Tech-2 goes with us!
The Tech-2 was made for use on autos from about 1991 through 2013 when it was replaced by the MDI later MDI 2 with GDS2 software running on a laptop. Quite unwieldly to use while driving. The Tech-2 can hang from the steering wheel. The final software update is 33.004 for North American autos and is stored on the included PCMCIA card.
Not all Tech-2s are or were made the same. OTC, Bosch and Vetronix are the companies that I know of that were licensed to produce and sell these for GM. Vetronix being the last, had them produced in China. These latter ones are missing some of the features of the earlier ones like the ability to copy PCMCIA cards and the training emulator. Not really necessary for the home user. People say there are clones made but I believe that the factory in China kept making them and started selling them on their own when GM moved to the other platform. Does that mean the Chicoms stole it or did they change the design 10% to avoid copywrite violations? I don’t know the answer to those questions. However, mine (circa 2017) looks identical to a Vetronix unit made in China in 2010.
These things come with a bunch of cords and adapters that most will use but once for self-testing the first time out. The Candi Module is not needed for GMT800s and earlier. It’s nice to have the various power adapters. Mine came with a bad OBDII adapter but once discovered, was promptly replaced. I bought the hard case made for it; keeps everything snug as a bug in a rug. Some ads say not for trucks. I can only think they mean the Kodiak/Top Kick and the White GMC types.
Often the Tech-2 will come with TIS2000 software. If your vehicle was made before 2009 or so then you would probably not have to use the pay to play Tis2Web service. A recorded data log can be downloaded to the laptop and played through this software, which makes it much easier to see all the parameters moving at once. Oh, and it can work with Windows 7 too. Some videos are posted on my YouTube channel.
The following is how the Tech-2 has benefited our family. Ever since buying the truck the gas mileage was off, not good. Even after having it tuned by BlackBear and having him disable AFM the mileage slowly got progressively worse. Finally, a code started popping up for a bad catalytic converter. People would swear it was the O2 sensors but the Tech-2 and shop manual teach you how to test those out. It was the catalyst. Then a few months later the other one went and the whole Y-pipe was replaced with a new one. To start out fresh, the Tech-2 was used to reset the Fuel Trims and Alcohol Content. The Tech-2 is also used to record the alcohol content of the fuel as it mostly runs on E85. Some gas stations are sneaky and put a wee bit too much ethanol in their gasoline.
During the summer I noticed that the driver’s side air wasn’t getting as cold as it used to. Tech2 showed a code and the reason why for the driver’s side actuator. For a year now, I have been able to command that actuator back into position and sync it, kicking the can down the road. After time, another actuator got stuck and the Tech-2 got it unstuck.
In the tire section, there are entries for the type of tire and air pressures run. My truck originally came with P-Metric tires with the air pressure set for 30 psi. Using the Tech-2, we told the truck’s computer that it is now wearing Load Range C LT tires at 50 psi. This means the low-pressure warning will now come on at 40 psi instead of 20.
The Tech-2 of course can see which TPMS sensors are at which wheel. This has exposed a tire store who said they rotated the tires when they didn’t! The new positions are written down after each rotation.
The truck came with a basic ACDelco silver battery. Watching the parameters in the Body Control Module, I can judge its health and watch the system carefully charge it up and shut down the alternator when it is fully charged, or at the point the computer wants it to be at.
One day a scan turned up a bad coax cable for OnStar. Phone still sounds good. Later on, sometimes, the OnStar light would be red for a few seconds and go green on its own or with the help of the Tech-2. About a year later my Bluetooth calls would sometimes get a lot of interference. The other coax had thrown a code. Sealing the shark fin antenna has helped a little. GM will get to do that repair, I’m not dropping the whole headliner to change those out! But at least I know what is wrong and how it needs to be fixed. No smoke and mirrors should get past me.
Well, I hope this post proves helpful and saves you and your family some money when it comes time for repairs and maintenance. I have no formal training with these and have probably only scratched the surface as to what they can do. I hope some GM Techs will chime in.
Did you know that the GMT-900s have an internal transmission oil life monitor that can be reset? Now, I bet you want to know what your oil’s life is, right? Get a Tech-2!
December 2018

Put a 4" crank in an L96 and you've got 402 cubic inches and with the right tuning could easily make about 500 foot pounds of torque.
Cliff, you're running that big truck for less than my daughter's Kia at twelve cents a mile! I was getting close but they're back up to around $2.25 a gallon and much more for it around here.